The making of accurate test specimens is essential to the construction industry. Ideally, the test specimen would have the identical properties of the main structure so that results of tests performed on the specimen could be attributed directly to the main structure or extrapolated by known formulas to accurately predict the properties of the main structure. In practice, the attainment of identical properties is extremely difficult especially if the volume difference between the main structure and the test specimen is large. In the construction industry, this volume difference is usually extremely large.
Concrete is a widely used material in the construction industry and there are a number of factors that affect its ultimate strength. Some of these factors are mechanical such as the materials used, the mix proportions, and the extent to which the concrete is compacted and some of these factors are chemical such as the time-temperature curve of the curing process. The reaction of the concrete mixture is itself exothermic and it is the practice in many fields of the concrete industry to accelerate the curing process by heating the skins of the forms. Normal curing usually has a time-temperature curve that is relatively flat during the initial pre-set hours and then rises for a few hours. If the mold is insulated, this rise can be on the order of 30.degree. F. above the pouring temperature. With accelerated curing, heat is added to the concrete in the form to reduce the overall time of the curing process. This practice is particularly common in making pre-cast and pre-formed concrete. In one such process, live steam, hot water, hot oil, or other hot fluid is passed through a system of pipes positioned near the skin of the form. The skin of the form is then actually heated by energy radiating from the pipes. A less common procedure is to place an electric heating unit against the skin of the form. In these accelerated curing procedures, the temperature of the concrete in the form is raised after the initial pre-set period of 1-3 hours up to 135.degree.-160.degree. F. in 2-3 hours (20.degree.-40.degree. per hour) and maintained at the elevated temperature for about 8 hours. In accelerated curing and normal curing, a vapor shield is usually placed about the form so that the concrete will not dehydrate.
In making test specimens, accurately duplicating the time-temperature curve of the curing process of the main structure is as important, if not more important, than accurately duplicating the materials, mix, compaction, and other mechanical features. One technique for trying to match the two curves when the main structure is curing without the addition of external heat is to place the mold for the test specimen as near to the main structure as possible. This procedure for making test specimens has proven to be most unsatisfactory regardless of how close the two are placed and regardless of whether the main structure and mold are insulated or not. One problem with this procedure is that the ratio of the volume of the test material to the surface area of the mold is much smaller than the ratio of the volume of the main structure to the surface area of its form. Consequently, the mold affects the temperature of the specimen much more than the form affects the temperature of the main structure. Further, the mold is often constructed from a different material than that of the form so that the heat transferring properties are different. Due to these factors and other factors, the time-temperature curve of the test specimen in the mold is often much different from that of the main structure. In practice, it has become necessary in many cases to add external heat to the mold of the test specimen in order for it to match the temperature created in the main structure, expecially if the main structure is insulated. If the curing of the main structure is accelerated, it is mandatory that the test specimen also be externally heated.
One known manner of substantially matching the time-temperature of a main structure is to place the test specimen and molds in a liquid whose temperature is controlled according to the temperature of the main structure as disclosed in the British Pat. No. 1,300,099 to Thompson issued on Dec. 20, 1972. This technique has several drawbacks. One disadvantage of Thompson's system is that it is bulky and would be difficult to work with in the field. Another disadvantage is that Thompson senses the water temperature around the molds rather than the temperature of the test specimen itself or the temperature of the mold's surface immediately adjacent the test specimen. He also heats the water which heats the mold rather than heating the mold directly. Thompson's heat must pass from the water through the mold to the test specimen. Since Thompson adds his heat to the water and controls this addition of heat by monitoring the temperature of the water, he has the further problem of uniformly delivering the heat to the molds without the establishment of convection currents and temperature gradients in the water that would affect the accurate delivery of heat to his molds and the accurate reading of the water temperature. This problem is particularly acute if the water volume is relatively large.
Apart from the problem of accurately heating the test specimen within the mold, the mold itself should have certain characteristics of its own. It should be easily formed or cast, long-lasting, and easy to operate. For ease of forming or casting, it is best to have a mold whose halves are substantially identical so that they themselves can be made from the same mold. To be long-lasting, the mold should be strong without being unduly heavy and have as few moving parts as possible as well as features to protect the areas most vulnerable to wear and abuse. For ease of operation, the mold should also have as few as possible moving parts that must be manipulated during the molding process. Further, the halves of the mold should be able to be quickly aligned as the mold closes and to be quickly and easily screwed together. The mold should also have an arrangement for initially prying the halves apart to remove the test specimen, an arrangement for stopping the movement of the mold halves relative to each other at a pre-determined open position, and an arrangement for removing a test specimen that may become fixed to the mold surface of one of the mold halves. This arrangement for removing or stripping a test specimen from fixed engagement with one of the mold halves should do so without disturbing the boundary areas including their edges that are necessary for proper testing of the specimen. In this regard, the symmetry of the mold halves is also desirable to avoid localized stressing of the test specimen and damage to the boundary areas as the mold is opened. In addition to these characteristics of the mold, the mold must be water tight if it is to meet ASTM standards.
Numerous U.S. Patents illustrate molds, however, none of these is known to have the symmetry, ease of operation, or durability of the present invention. Examples of molds whose halves are pivotally mounted to each other include U.S. Pat. No. 166,667 to Watkins issued on Aug. 10, 1875, U.S. Pat. No. 1,739,769 to Redmann issued on Dec. 12, 1929 and U.S. Pat. No. 1,927,717 issued to Rothmann on Sept. 19, 1933. Among other things, these patents lack symmetry and ease of production and operation. Watkins, for example, has an asymmetric pivot arrangement as well as asymmetrically positioned securing means. U.S. Pat. No. 1,533,341 to Rodler issued on Apr. 14, 1925 illustrates a mold with aligning pins 11 for the mold halves and securing members 7-10. Rodler's mold halves 1 and 2 are not pivotally mounted to each other and he has no means for initially prying the mold apart. U.S. Pat. No. 2,986,797 to Aisenberg issued on June 6, 1971 also shows the use of aligning pins 36 which are located between the ends of the mold halves like those of Rodler. U.S. Pat. No. 3,454,257 to Dupuis issued on July 8, 1969 and U.S. Pat. No. 2,974,385 to Leisenring issued on Mar. 14, 1971 illustrate asymmetric molds which are opened and closed from only one side. Such molds have the problem of losing their symmetry after numerous uses as well as during use (see lines 33-44 of Dupuis' column 3). Such molds also have the problem of stressing the test specimen and perhaps scarring the boundary areas of the specimen when the mold is opened. Their lack of symmetry also makes them somewhat difficult to operate quickly. An example of a mold whose halves are mounted for sliding movement relative to each other is illustrated in U.S. Pat. No. 812,935 to Knapp issued on Feb. 20, 1906.
The ideal mold for making test specimens of concrete or other material would be easy to make, durable, easy to operate, and able to accurately follow the temperature variations in the main structure. The present invention offers such a mold.